This invention relates to a piezo-electric oscillator, more particularly a piezo-electric oscillator provided with an automatic gain control circuit capable of stabilizing the output voltage and reducing the power consumption of the oscillator.
Conventionally, a piezo-electric oscillator has been proposed which comprises in combination an automatic gain control circuit including a field effect transistor, as shown in FIG. 1 of the accompanying drawing and fully described in Japanese patent publication No. 24066/1975.
In the prior art circuit shown in FIG. 1, a Colpitts type crystal oscillation circuit is constituted by a quartz oscillator element X, serially connected capacitors C.sub.1 and C.sub.2 and a transistor Tr.sub.1, and the DC operating point of the oscillation circuit is determined by bias resistors R.sub.1 and R.sub.2 and an emitter resistor R.sub.3. A DC voltage obtained by amplifying the emitter voltage of the transistor tr.sub.1 by an AC amplifier AMP and then rectifying the output thereof by a diode D.sub.1 is applied to the gate electrode G of a field effect transistor Tr.sub.2 to act as a control voltage. Since the resistance between the source and drain electrodes S and D, that is, the internal resistance of the field effect transistor Tr.sub.2 varies depending upon the bias voltage across the gate and source electrodes applied by a resistor R.sub.4 connected therebetween, this internal resistance varies and particularly increases as the DC control voltage from the diode D.sub.1 increases. Since the output of the oscillation circuit is zero at the time of starting oscillation, the D C control voltage is also zero and consequently the internal resistance of the field effect transistor becomes small, thus increasing the oscillation amplitude. As the oscillation amplitude increases, the DC control voltage also increases to increase the internal resistance of the field effect transistor, thereby automatically controlling the amount of feedback in a manner to obtain a steady state oscillation amplitude.
However, since the amount of feedback is controlled by a parallel circuit comprising the inductance of a coil L connected to the collector electrode of transistor Tr.sub.1 and the internal resistance R of the field effect transistor Tr.sub.2, the feedback impedance Z and its phase angle .theta. are given by the following equations: ##EQU1##
In this manner, the phase angle .theta. of the feedback signal varies depending upon the internal resistance of the field effect transistor. Accordingly, although the oscillator of the type shown in FIG. 1 can automatically control the oscillation amplitude, there is a defect in that its oscillation frequency varies.
Moreover, in the FIG. 1 oscillator, in order to stabilize the output voltage with respect to the power source voltage, it is necessary to stabilize the voltage across bias resistors R.sub.1 and R.sub.2 of the oscillation circuit and the bias voltage applied to the AC amplifier AMP, requiring the provision of a separate voltage stabilizing circuit. However, the use of the AC amplifier and the voltage stabilizing circuit not only increases the number of the component parts but also increases the power consumption.